Amazon is among several companies to have expressed interest in acquiring popular enterprise collaboration app maker Slack, with any deal tipped to place a value of at least $9 billion on the company, Bloomberg reported.

Nothing is confirmed, though, and talks may not go further, the report said. Slack and Amazon declined to comment on the matter.

In May, Slack shrugged off concerns about the UK’s pending exit from the EU by opening a new office in London. In April 2016, Slack raised $200 million and was valued at $3.8 billion. Since launch, it raised a total of $540 million.

Slack has 5 million daily active users – 1.5 million of whom pay to use the service – and had $150 million in annual recurring revenue as of 31 January, Bloomberg said.

The app maker’s technology works with several services including Salesforce and Microsoft’s Skype so employees can do all their work within Slack.

Meanwhile, Microsoft is reportedly working on Skype Team following a failed $8 billion bid for Slack, and Facebook’s Workplace app is said to go head-to-head with Slack.